In the production of steel product, such as billets, slabs and blooms, by the continuous casting method, molten steel is poured into a mold from the bottom of a teeming vessel such as a ladle or a tundish that is operably positioned above the mold. In order to control product quality it is necessary that the level of the molten metal in the mold be maintained substantially constant. For this reason, it has been the practice to interpose a metering nozzle in the pour passage between the teeming vessel and the mold. Such metering nozzles are normally replaceably disposed in the bottom of the vessel and consist of a bore including an upper portion formed with either a conically tapered or spherically formed wall that terminates at its bottom in a flow restricting orifice formed of a diameter to produce the desired rate of molten metal flow into the mold.
In the continuous casting of steels deoxidized by aluminum, magnesium, titanium or rare earth compounds, collectively referred to herein as "aluminum-killed" steels, there is a tendency for alumina or other refractory oxide inclusions of microscopic proportions to accumulate in the pour passage through the metering nozzle. These inclusions accumulate in the flow restricting orifice causing the flow opening to constrict thereby adversely affecting caster operation by initially requiring the speed of the caster to be reduced in order to maintain the required metal level in the mold, and finally, requiring replacement of the orifice when the constriction becomes excessive. If the nozzle is fixed in the teeming vessel, the casting operation must be terminated in order to replace the nozzle. If the nozzle forms part of a sliding gate valve, several gate changes may be required during the pouring of a single cast in order to replace the contained nozzles.
The problem is more acutely manifest in low production facilities, i.e., those in which caster operation compels the use of metering nozzles containing orifice openings of less than one inch diameter. In these facilities the production of aluminum-killed steel product in a continuous casting operation cannot be accomplished on a practical commercial basis due to the rapid obstruction of the small diameter orifice openings and the attendant frequency of nozzle replacements that result.
It has been contemplated to alleviate the described problem by cementing a thin disc containing the flow control orifice in the bottom of the pour opening of the teeming vessel. Such a device, known as a "wafer nozzle", is shown and described in Japanese patent application Ser. No. 24208/72 which was laid open on Nov. 11, 1973, under Ser. No. 92226/73. Applicants have found, however, that "wafer nozzles" are not totally dispositive of the problem for several reasons. First, such devices which are normally formed of a refractory material of greater density than the surrounding refractory of the vessel lining are prone to rapidly deteriorate due to the imposition thereon of high thermal stresses. Secondly, the flow of metal through these nozzles is often disturbed, principally as a result of turbulent fluctuations in the molten metal bath within the teeming vessel, thereby producing instability of the flow stream through the orifice opening and flaring in the stream emerging therefrom. An unstable flow stream gives rise to some, albeit reduced as compared with other devices of the prior art, inclusion accumulation on the nozzle. Moreover, both of these characteristics present the danger of causing metal oxidation due to increased exposure of the metal to air.
It is to the alleviation of this problem, therefore, that the present invention is directed.